Tuesday, October 07, 2014

Are lanes on urban streets too wide?

The typical width of a lane on a highway or freeway is twelve feet. That might be fine for higher speed traffic in rural areas, but Jeff Speck argues that it's too wide for streets in urban areas, and that urban traffic lanes should be no more than ten feet in width. Speck points the finger at traffic engineers, who have designed urban streets using geometries meant for highways because they think it is safer for motorists:
Why do they do this? Because they believe that wider lanes are safer. And in this belief, they are dead wrong. Or, to be more accurate, they are wrong, and thousands of Americans are dead.

They are wrong because of a fundamental error that underlies the practice of traffic engineering—and many other disciplines—an outright refusal to acknowledge that human behavior is impacted by its environment. This error applies to traffic planning, as state DOTs widen highways to reduce congestion, in compete ignorance of all the data proving that new lanes will be clogged by the new drivers that they invite. And it applies to safety planning, as traffic engineers, designing for the drunk who's texting at midnight, widen our city streets so that the things that drivers might hit are further away.

The logic is simple enough, and makes reasonable sense when applied to the design of high-speed roads. Think about your behavior when you enter a highway. If you are like me, you take note of the posted speed limit, set your cruise control for 5 m.p.h. above that limit, and you're good to go. We do this because we know that we will encounter a consistent environment free of impediments to high-speed travel. Traffic engineers know that we will behave this way, and that is why they design highways for speeds well above their posted speed limits.

Unfortunately, trained to expect this sort of behavior, highway engineers apply the same logic to the design of city streets, where people behave in an entirely different way. On city streets, most drivers ignore posted speed limits, and instead drive the speed at which they feel safe. That speed is set by the cues provided by the environment. Are there other cars near me? Is an intersection approaching? Can I see around that corner? Are there trees and buildings near the road? Are there people walking or biking nearby? And: How wide is my lane?

All of these factors matter, and others, too. The simplest one to discuss, and probably the most impactful, is lane width. When lanes are built too wide, many bad things happen. In a sentence: pedestrians are forced to walk further across streets on which cars are moving too fast and bikes don't fit.
Speck goes on to lay out the case for narrower traffic lanes on city streets. He points out that the AASHTO "Green Book," which guides the geometric design of streets and roadways, says that 10-foot lanes are acceptable in urban areas, and cites studies showing that narrower lanes are no more dangerous, and in some cases safer, than standard 12-foot lanes in terms of accident rates. He argues that wider lanes cause motorists to drive faster, which results in accidents that cause more injuries and deaths than accidents that occur at lower speeds. Speck maintains that narrower lanes do not impede traffic flow in urban areas and that re-striping urban streets from 12-foot lanes to 10-foot lanes will make them safer for pedestrians as well as create enough extra space for buffered bike lanes.

Speck's arguments are ones I have heard before and generally agree with. In my experience, however, the most ardent proponents of wider lanes are not traffic engineers, but fire departments, who insist that their apparatus can only be safely handled by 12-foot lanes (and indeed, the comments to Speck's article make note of this). There's also the issue of semi trucks and buses being able to safely turn from narrower lanes. And, to be honest, I sometimes feel more comfortable by the extra space that those 12-foot lanes provide between myself and the moron in the Tahoe or F-250 next to me who is talking on his cell phone and not paying attention to the guy in the lane next to him. 

Which brings up a point: at the end of the day drivers are responsible for their own behavior, including the ability to safely navigate down streets, regardless of lane width.

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